“Manufacturing-side” means that the calibration is performed at the manufacturer's location as a part of the fabrication, or production, of the measuring device. The capacitive method permits a continuous measurement of fill level. In such case, the probe-unit and the container wall (or a second probe-unit) form a capacitor, whose dielectric is the medium in the container. The capacitance of this capacitor is, in such case dependent on, among other things, the fill level of the medium, so that, from the capacitance, the fill level can be deduced. Various possibilities for measuring the capacitance are set forth in, for example, the Offenlegungsschrifts DE 01 57 762 A1 or DE 101 61 069 A1 of the assignee.
Problematic is the fact that the measured capacitance depends not only on fill level but also on the dielectric constant and conductivity of the medium. Since the conductivity changes e.g. as a function of temperature, or the humidity of the air, measurement uncertainties, or limitations as regards application, result. Additionally, also the geometry of the container and, for instance, accretions on the probe unit, affect the measured values.
Another difficult area, in view of the multiple dependencies of the variables of concern, is the correlating of the capacitance value determined from the measurement and the matching fill-level value actually of interest. Consequently, usually a so-called calibration is required, in which, following installation of the measuring device, various fill levels are brought about, using the medium to be measured, and the capacitance values resulting from such fill levels are recorded. Such a calibration, however, involves much effort and stands in the way of a direct start-up of the measuring device following installation.